New diseases of Trillium in the UK caused by Colletotrichum lineola and Urocystis trillii

Trillium species consist of temperate-forest herbs and are native to North America and Asia (Case & Case, 1997). They are spring-flowering perennials with three sepals and three petals. In the UK, they are specialist grown plants suitable for the woodland garden. In May 2005, Trillium erectum f. albiflorum diseased plants originating from a nursery in Hampshire were received at the Royal Horticultural Society. Symptoms included pale brown lesions on the stems and leaves in which dark setae were visible (Fig. 1). Conidia of the fungus were hyaline, aseptate with an acute apex and truncate base and measured 21-29 x 2.9-1.6 μm (Fig. 2). Setae were usually one-septate at the base, brown and measured 123-400 μm (Fig. 3). The morphological characteristics fitted a Colletrotrichum species. As there is a high variability of the morphological features in Colletotrichum, the identification was achieved by sequencing of the ITS and the histone3 (HIS3) regions (Damm et al., 2009). The ITS and HIS3 regions were amplified using ITS1/ITS 4 primers (White et al., 1990) and CYLH3F/CYLH3R primers (Crous et al., 2004) respectively and sequenced (GenBank Accession Nos. HQ239359, HQ239360). Although the ITS sequence was identical to Colletotrichum dematium and C. linolea, the H3 sequence differentiated both species and confirmed the identity as C. lineola. An herbarium sample was deposited at the Royal Botanic Gardens, Kew (K(M)164723). 

The fungus was isolated on PDA and sporulated after two weeks at 20°C under a 16 h photoperiod. To fulfil Koch's postulates, Trillium grandiflorum plants were dipped in a suspension of ~ 2 x 10⁶ spores/ml and covered with a plastic bag for 48 hrs to maintain a high humidity. The plants were left outside where the temperature ranged from 12° to 25°C. After two months the plants collapsed and the fungus was re-isolated from the inoculated tissues. Control plants were water-inoculated and remained healthy. 

In June 2010, Trillium kurabayashii plants originating from Norfolk were received at the RHS showing typical symptoms of a smut disease (Figs. 4A & B). Sori exposing black spore masses were observed mainly on stems sometimes spreading to the base of leaves. The sori were large and confluent with individual sori reaching 3.5 cm in length. Ellipsoid, ovoid brown spore balls measured 29-42 x 29-58 μm and were surrounded by a layer of hyaline sterile cells measuring 8-12 x 5-9 μm. The spores were chestnut-brown ovoid or polyhedral and measured 14-23 x 11-17 μm (Fig. 5). The morphological characteristics fit the description of the smut fungus Urocystis trillii (Jackson, 1920). An herbarium sample was deposited at the Royal Botanic Gardens, Kew (K(M)166157). The ITS region was sequenced as above (GenBank Accession No HQ239361) and was unique when compared to other sequences in GenBank.

Colletotrichum lineola is a widespread pathogen affecting a wide range of plant species in temperate regions. The only occurrence of the fungus on Trillium was reported from the USA (Farr & Rossman, 2010). Urocystis trillii is a rarely reported smut specific to Trillium that occurs in Canada, Japan and Western North America (Farr et al., 2010). This is the first case of the disease in the UK or indeed Europe.